Johnston, Morphology of the Head. 251 



of the specialized nucleus known as the epistriatum is a difficult 

 question. It serves in fishes as the end nucleus of two very 

 different classes of fibers. The one comes from the inferior 

 lobes and corpora mammillaria by way of the anterior commis- 

 sure; the other comes from the cells of the olfactory bulb 

 through a somewhat isolated portion of the anterior commissure. 

 It is probable that this is the more primitive tract and that it is 

 gradually supplanted by tracts from the h)^pothalamus and per- 

 haps other regions. In any case the epistriatum seems to be- 

 long chiefly to the visceral sensory apparatus. 



The origin and relationships of the cerebral pallium have 

 been discussed by the writer elsewhere (70). The massive 

 pallium which is related to the olfactory apparatus is regarded 

 as a neomorph in gnathostomes and as such it is not of especial 

 importance for our present subject. It has been suggested 

 above, however, that this olfactory pallium may have been 

 preceded by a structure which was the general cutaneous 

 nucleus of the first segment. The pallium, then, is to be re- 

 garded as a new structure developed in the space from which 

 the lateral eyes were derived. 



If Hill's interpretation of the neuromeric relations is cor_ 

 rect, we are brought to the interesting result that the dienceph- 

 alon is formed from two neuromeres, the hypothalamus belong- 

 ing to the same neuromere with the striatum and pallium and 

 lying morphologically ventral to them. The writer has shown 

 that in fishes (Acipenser, 6]) throughout the whole length of 

 the ventral wall of this second neuromere the structures adjacent 

 to the middle line (nucleus thaeniae, inferior lobes in part, and 

 corpus mammillare) are histologically identical. In Petromyzon 

 (68) the inferior lobes and corpus mammillare are fundamentally 

 alike, there being but slight differentation. It seems, then, that 

 a single nucleus has been interrupted by the optic chiasma and 

 accompanying decussations. The posterior portion of this 

 nucleus was associated with the saccus vasculosus and also came 

 to receive tertiary tracts from both the olfactory and the somatic 

 sensory centers. It has consequently developed into a large 

 coordinating center. The anterior portion of the nucleus (nuc. 



